An electromagnetic actuator is known, comprising two armatures and a magnetic circuit of which the reluctance is variable during operation between a minimum value attained at one end of the stroke and a maximum value attained at the other end of said stroke. In such an actuator, the electromagnetic control force increases continuously when passing from the end of the stroke where the reluctance of the magnetic circuit is maximum to the end where the reluctance is minimum; the moving part by means of which the control is transmitted is generally subject to a slight return force; it is thus observed that the control force available varies along the stroke in a continuous manner on which it is impossible to act. The value of the force available obviously depends on the electromagnetic coil forming part of the actuator and the idea has already been proposed of arranging, inside the same actuator, several distinct coils of which the supplies are arranged according to the position of the moving part of the actuator; in particular, the idea has been proposed of using two coils which act simultaneously to initiate travel of the moving element of the actuator but of which one remains supplied when the moving element reaches its position where the reluctance of the magnetic circuit is at a minimum. Such an actuator can be used, in particular, for controlling the travel of the starter pinion connected to the crown of a car starter motor.
However, electromagnetic actuators of known type have the basic disadvantage of delivering an available control force which varies in an unalterable continuous and defined manner along the stroke of the moving part of the actuator. This continuous variation may not be troublesome for certain applications but it constitutes an obstacle in other applications, particularly if an electromagnetic actuator is to be used for controlling mechanical engagement and disengagement of a motor vehicle clutch. In fact, it is known that the effort to be applied to the control diaphragm of the clutch in such a case is variable along the stroke according to a so-called "saddle-back" curve; this curve giving the force F as a function of the stroke C has a decreasing zone situated between two increasing zones. In other applications of actuators, it may be particularly worthwhile to keep the available control force constant over a large proportion of the stroke. It has thus been found desirable to be able to act on the form of the curve representing the variation in the control force supplied by the actuator as a function of the stroke of the moving element of the actuator.